This invention relates to the confinement of plasmas by magnetic fields and, more particularly, to an apparatus and method for the formation of a spheromak plasma.
Devices employed for the containment of plasmas by magnetic fields may have various configurations. Two well-known types of such devices are the open-ended type, such as the magnetic mirror type, and the toroidal type, such as the tokamak. The underlying principle of all types of such containment devices is the containing of a hot, dense gas away from physical walls for a time sufficient to allow fusion reactions to take place.
An advantage of the mirror-type device is that it has have a coil-blanket topology which does not link the plasma. However, the mirror-type open ended apparatus has a disadvantage in that the trapped charge particles may escape while travelling along the magnetic field lines which define their spiral orbits. The magnetic field lines do not close upon themselves inside the magnetic mirror, thus compounding the problem of large plasma losses through the mirror ends. It occurred to many people in the early days of fusion research that mirror end losses could be easily eliminated simply by bringing the two ends of the straight cylinder on themselves, thus forming the well-known torus device.
The toroidal-type devices have an advantage in that plasma is well confined in the closed magnetic field lines. Since the ions tend to remain in a spiral orbit about a given set of magnetic field lines, the continuity of the magnetic field lines inside the apparatus enhances containment. A tokamak has clearly this above mentioned advantage but suffers from a difficult topology in which the coil-blanket links the toroidal plasma.
The spheromak combines the most advantageous aspects of the above-discussed toroidal and mirror schemes. The spheromak is characterized by magnetic field lines which are closed, as in a tokamak, and by a coil-blanket topology which does not link the plasma, as in a mirror-type device.
Among the advantages of this spheromak formation scheme is the ability to keep the physical structure of the apparatus away from the plasma, thus reducing absorbed impurities and keeping the plasma "hot". Also, the spheroidal blanket simplifies the design and construction of the reactor apparatus. The PG,4 magnetic field configuration of the spheromak includes both toroidal and poloidal components, but the toroidal component is maintained entirely by plasma currents, and, therefore, it vanishes outside the plasma. The outward pressure of the toroidal field and of the plasma is balanced by the inward pressure of a poloidal field.
For additional background discussions relating to the spheromak configuration, the reader is referred to S-1 Spheromak, Princeton University, Plasma Physics Laboratory, Aug. 24, 1979, the disclosure of which is hereby incorporated by reference.
Configurations of the spheromak type were first studied theoretically in an astrophysical context by Lust et al (Z. Astrophysics 34 (1954) 263) and Chandrasekhar (Proceedings of the National Academy of Sciences 42 (1956) 1). More recently, an experimental investigation of the spheromak in the fusion context was carried out by Alfven (Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy 31 (1958) 3).
Extensive MHD equilibrium analyses have been carried out upon spheromak and spheromak-like configurations by Morikawa in Physics of Fluids 12 (1969) 1648; 13 (1970) 497; and 16 (1973) 140. However, each of the above references was primarily directed to the theoretical aspects of spheromaks, and was not concerned with how one would go about forming such a configuration.
More recently, Bussac et al (Plasma Physics and Controlled Nuclear Fusion Research, Seventh International Conference, Innsbruck, 1978) discussed the advantage of a spheromak configuration for use in a fusion reactor. Bussac, in a theoretical context, discusses the basic parameters for both large and small spheromak reactors, but only alludes to the fundamental problem of how such a spheromak configuration is to be formed, citing the heretofore known techniques discussed below.
Two known methods of spheromak plasma formation suitable for spheromak start-up have been experimentally confirmed. The first of these is the so-called "Marshall gun" approach, which is discussed in Alfven, Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy 31 (1958). This approach is characterized by the establishment of an initial poloidal field, followed by the application of toroidal flux through an electrode system. Plasma inertia is relied upon to immobilize the toroidal flux while the poloidal field lines are reconnected within the plasma. This approach has the disadvantage of requiring formation on a dynamic time scale, leading to questions of whether the internal poloidal flux is adequately reconnected. Also, since an electrode system is used, this formation scheme may suffer from problems of erosion and impurity influx, causing plasma cooling problems.
Another known scheme suitable for spheromak startup is the familiar reversed-field theta-pinch approach, as discussed in Centre de Recherches en Physique des Plasmas, Lausanne, Switzerland (1978-79). This scheme is quite similar to the Marshall-gun approach, and thus suffers from the same disabilities. The major difference between the two approaches is that the geometry of the plasma forming structure is rotated by 90.degree. relative to that of the Marshall-gun approach, thus producing radial, rather than axial, plasma acceleration.